Why is Tom Buchanan portrayed unfavorably?

Tom Buchanan is shown in a consistently unfavorable light throughout the novel. He has a beautiful wife, but he is blatantly unfaithful to her. Tom shows bad taste in the woman who is his current mistress. He takes pride in introducing Nick to her right under her husband's nose.
Myrtle Wilson is cheap, ignorant and vulgar but seems suited to her rich lover. She deceives her poor husband without qualms. The love and sincerity of George Wilson only make his wife look more ruthless. Tom Buchanan is a big, brutal man. He shows himself at his worst when he gets into a drunken quarrel with Myrtle at their New York "love nest" and breaks her nose with a blow of his open hand.
Fitzgerald's purpose in creating such an unfaithful, unattractive and unsympathetic husband for Daisy was to soften the culpability of Gatsby in intruding into the life of a married woman with a small child and trying to win her away from her husband. If Tom Buchanan were a better husband, it would make Gatsby look like more of an unscrupulous predator.
Both Nick Carraway and Jordan Baker see Tom as an ignorant man who knows nothing but polo. Tom is rich because he inherited all his money—and he is getting richer all the time because these are the Roaring Twenties and the stock market is soaring. He never earned any money in his life. Gatsby, by contrast, earned everything he has—although he earned most of it through clandestine enterprises with shady associates.